1. Field of the Invention
The present invention relates to a flame-retardant coated optical fiber, and specifically, to a flame-retardant coated optical fiber in which a projection of an optical fiber from its end face during actual use is very small.
2. Prior Art
Recently, there has been an increasing demand for securing flame-retardancy of various office automation apparatuses. Accompanying this trend, positive flame retardancy has been increasingly required of coated optical fibers which are used for the wire arrangement of these apparatuses.
As shown in the sectional view of FIG. 1, a typical coated optical fiber comprises an optical fiber 1, a primary coating layer 2 covering the fiber 1, and a secondary coating layer 3 covering the layer 2.
In order to obtain a flame-retardant version of this coated optical fiber, according to a generally known arrangement, a self-extinguishing thermosetting silicone rubber is used as a material of the primary coating layer 2, for example.
By means of the coated optical fiber which primary coating layer consists of the thermosetting silicone rubber, however, it is hard to enhance its flame retardancy to a level standing the VW-1 test prescribed by UL1581.
Accordingly, a proposed flame-retardant coated optical fiber is such that the thermosetting silicone rubber is used for a primary coating layer. And this coating layer is covered with a secondary coating layer which consists of a plastic material blended with a brominated flame retardant such as decabromodiphenyl oxide or tetrabromophthalic anhydride. This coated optical fiber exhibits an appreciable flame retardancy, due to a synergistic effect produced by the self-extinguishing property of the thermosetting silicone rubber for the primary coating layer and the good flame retardancy of the brominated flame retardant in the secondary coating layer.
In the coated optical fiber covered by the primary coating layer consisting of the thermosetting silicone rubber, however, the adhesion of the secondary coating layer to the primary coating layer is insufficient. In some cases, therefore, the optical fiber, as a core of the coated optical fiber, may project from the end face of the coatd fiber if it is repeatedly subjected to temperature change, thereby damaging a light source which is connected directly to the optical fiber.